Cellular communications traditionally used 2G and 3G technology, which communicate using a circuit-switched (CS) network to provide voice services and low speed data services and packet-switched networks to for high-speed data services. In cellular communications, the traditional networks are slowly being replaced by long term evolution (LTE) networks, which in embodiments communicate using 4G technology. In contrast to a CS network, an LTE network is a purely packet-switched network and does not inherently support voice services. The transition from a full CS network to a full LTE network is slow coming, and currently, there are circumstances when it may be advantageous to communicate via a CS network while at other times, it may be advantageous to communicate via an LTE network. As such, there is a desire in the industry to implement infrastructure changes that allow a mobile device to switch back and forth between a CS network and an LTE network such that the mobile device can communicate using either of the networks, as is desired.
A number of methods have been proposed to providing voice communications in the LTE networks. If support for voice communications is not available over the LTE network, service providers leverage existing CS networks to provide voice services. Therefore interworking solutions are needed to enable interworking between the LTE and the existing CS networks. One of the interworking solutions is the Single Radio Voice Call Continuity (SRVCC) defined in the 3GPP standard TS 23.216, wherein an LTE voice call is handed over to a 3G network whenever LTE coverage is not available for mobile devices in call. Another interworking solution is called Circuit Switch Fallback (CSFB) for example, as detailed in 3GPP standard TS 23.272. In this approach, the mobile device may register with the CS network after attaching to the LTE network. For voice services, the mobile device may be redirected or handed over to the CS network from the LTE network. For redirection to work, it is a pre-requisite for the mobile device to be attached in the LTE network as well as registered in the CS network.
In the industry today, mobile devices often have dual functionality such that they are configured to operate in both the LTE network and the CS network. Generally, the LTE network is used for data communications while the CS network is used for voice communications. As such, when a mobile device is sending uplink and/or downlink data transmissions, it is preferable that the mobile device communicate using the LTE network. Further, when a mobile device is sending uplink and/or downlink voice transmissions, it may be preferable that the mobile device communicate using the CS network. In operation, mobile devices tend to transmit more data communications than voice communications. For example, users tend to browse the internet and utilize apps more than they talk on the phone. To accommodate this typical use of the mobile device, many mobile devices are set to default to the LTE network. In such embodiments, the mobile device may switch to the CS network (e.g., perform a CSFB) when a voice communication is desired.
For example, a mobile device may be configured such that its default network is an LTE network. When a short message service (SMS), is sent to the mobile device, the mobile device is likely already on its default LTE network. As such, because the mobile device is already on the LTE network, the mobile device does not need to switch networks in order to receive the SMS. Likewise, when the mobile device wants to send an SMS, because the mobile device is already on the LTE network, the mobile device does not need to switch networks in order to send the SMS.
However, because the mobile device's default network is the LTE network, the same cannot be said when the mobile device wants to originate or receive a voice communication (e.g., a phone call). When a user wants to originate a voice communication (e.g., make a phone call) using a mobile device, the user will indicate to the mobile device that voice communication is desired. For example, the user may input a phone number and touch a send input. For mobile devices that support LTE and CSFB for voice services, when a user originates a call, the mobile device sends the request to the LTE network with an indication that it supports CSFB. Upon receiving such a request, the LTE network directs the mobile to connect with the CS network to make the call.
For mobile terminating calls, a mobile device may be paged in the LTE network and upon the mobile device responding to the page, the mobile device may be directed by the LTE network to switch to the CS network (e.g., CSFB) for the call. In order to support CSFB operation, industry has developed two different methods: (1) Mobile Switching Center (MSC) upgrading, which upgrades the existing MSC to support communication with the Mobility Management Entity (MME) in the LTE network and (2) virtual MSC, which involves a Circuit Switch Fallback Inter-working Function (CSFB IWF) supporting the communication with the MME and acting like a Mobile Switching Center/Visitor Location Register (MSC/VLR) in the CS network. The CSFB IWF solution requires no upgrades to the existing MSC.
The Mobile Switching Center (MSC) upgrading method developed as follows. A MSC is a legacy switching center, which is operable in a CS network. Most MSCs in operation today were designed and installed prior to the development of the LTE network and prior to CSFB. As such, legacy MSCs are not equipped with the hardware and software necessary to locally perform CSFB. Recently, there has been a move by MSC vendors to replace old MSCs with new MSCs that have hardware and software configured to locally perform CSFB. Further, some MSC vendors have developed upgrading packages which upgrade the hardware and/or software of legacy MSCs, thereby enabling them to locally perform CSFB.
The methods used to replace and/or update the MSCs have been sufficient in locally performing CSFB; however, the endeavor is costly. The amount of programming and cost of hardware causes this upgrade method to cost between half a million to one million dollars per MSC unit. As such, because many network providers utilize multiple MSCs in their networks, updating legacy MSCs may not be a viable option for many service providers.
Accordingly, because industry was looking for an alternative solution to MSC upgrading, other vendors developed a centralized solution involving virtual MSC. In this solution, a new device was added to the network which communicated with several legacy MSCs and had the functionality needed to inform a mobile device when CSFB may be desirable. The new devices, which were added to the networks, were unique versions of a visitor location registration (VLR) and were sometimes called a voice service gateway (VSG). A single VSG could provide the desired functionality in a centralized location on behalf of several MSCs thereby minimizing the cost of implementation. However, VSGs were not embraced by network providers because in operation, the VSGs have proven to add additional delay to the call set up time and has not met mobile user expectations.
The virtual MSC method employed by the VSGs operated to inform a mobile device that an inbound voice communication was destined for the mobile device. The inbound call was first received at an MSC of the CS network, and the MSC was tasked with finding the mobile device so that the MSC may deliver the voice communication to the mobile device. According to the method, the MSC (e.g. gateway MSC (gMSC)) queried a home location registration (HLR) of the CS network to request routing information for use in routing the incoming voice communication to the mobile device. In this method, the HLR then queried the VSG which was configured to communicate with devices in both the CS network and the LTE network. Because the VSG could communicate with both networks, the VSG could receive a query from the HLR (of the CS network) and assist in locating a mobile device on the LTE network.
Upon the VSG receiving the query from the HLR, the VSG located the mobile device as being attached to the LTE network and determined which mobility management entity (MME) of the LTE network was in communication with the mobile device. Upon determining which MME was in communication with the mobile device, the VSG sent a request to the MME requesting that the MME send a page to the mobile device informing the mobile device that a voice communication was inbound for the mobile device. Due to the page, the mobile device was able to determine that it may be desirable to switch back to the CS network. During and/or after switching back to the CS network, CS network and the mobile device performed operations to register the mobile device with the CS network. If the mobile device was able to successfully switch back to the CS network and properly register to the serving MSC (sMSC) with the CS network, the mobile device informed the MME of the successful CSFB and the MME relayed this information to the VSG.
Further, after the VSG determined that the mobile device had successfully switched back to the CS network and was properly registered to Serving MSC (sMSC) with the CS network, the VSG then queries HLR (second query) for the actual mobile device location. During this second query, HLR asked the sMSC to provide mobile station roaming number (MSRN) such that the call can be routed to sMSC for terminating the call to the mobile device. When sMSC receives the request and responds back the MSRN to the HLR, then to the VSG. Then VSG was able to respond the initial request by using the MSRN received from sMSC to the HLR, and the HLR was able to answer the gMSC's query by sending the MSRN to gMSC. With the mobile device's routing number, the gMSC sent the voice communication to sMSC to terminate the call to the mobile device.
While, the above method enabled mobile devices to determine when it may be desirable to switch back to the CS network and enabled the inbound voice communication to reach the mobile device, all steps of the above method were performed in the time period after the inbound call was placed (e.g., after the caller placed the call) but before the mobile device indicated to the user that a call was incoming (e.g., before the callee's phone rang). Performing all of the steps after the inbound call was placed added a delay of 3-5 seconds, which was discovered to be unacceptable to the service provider's customers. Further, because the mobile device's default network is the LTE network, after the voice communication was concluded (e.g., after the call was finished), the mobile device automatically switched back to the LTE network. As such, the steps involved in the virtual MSC method were performed for each and every voice communication that was inbound for the mobile device. Thus, every inbound call experienced the aforementioned 3-5 second delay, which made the method further unacceptable to the service provider's customers.
Because the service provider's customers were not satisfied by virtual MSC solution, the solution was not accepted by the industry, thereby leaving the industry with the previously described MSC upgrading solution. However, as explained, due to the cost prohibitive nature of the MSC upgrading solution, a new solution which resolves the time delay created by Virtual MSC while minimizing the costs associated with MSC upgrading is desired.